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Contents lists available at ScienceDirectOptics and Laser Technologyjournal homepage: www.elsevier.com/locate/optlastecResearch NoteUnconventional magnif i cation behaviour in microsphere-assistedmicroscopyStephane Perrin ? , Hongyu Li, Sylvain Lecler, Paul MontgomeryICube Laboratory, University of Strasbourg – CNRS, 67412 Illkirch, FranceH I G H L I G H T S•Magnif i cation in microsphere-based microscopy behaves dif f erently than in optical microscopy.•Lateral magnif i cation increases along the microsphere imaging depth.•Magnif i cation factor depends on the microsphere size and illumination conditions.•Reducing the spectral bandwidth of the light source narrows the magnif i cation range.A R T I C L E I N F OKeywords:MicroscopySuper-resolutionMicrosphereMagnif i cationImage formationA B S T R A C TMicrosphere-assisted microscopy is an original sub-dif f raction-limit imaging technique allowing to reach a fewhundred nanometres of lateral resolution in air only by placing a microbead in a classical optical microscope.This work aims to highlight the magnif i cation process in microsphere-assisted microscopy which behaves dif-ferently from the case in optical microscopy. As a matter of fact, the lateral magnif i cation of an optical mi-croscope does not change according to focus plane positions. Experiments on the super-resolution imagingtechnique, performed in air through soda-lime-glass microspheres and attested by simulations, demonstrate asignif i cant increase in the magnif i cation factor along the microsphere imaging depth, i.e. at dif f erent object-focal-plane position of the objective. Moreover, it is shown that the magnif i cation range, as well as its slope,depend on the size of the microsphere. Additionally, the inf l uence of the spectral width of the illumination lightsource on the magnif i cation range is highlighted.1. IntroductionResolving power in classical optical microscopy is limited by dif-fraction of light, resulting in a minimal distance between two distinctobject details of half of the wavelength in air using a low-coherent il-lumination [1]. The idea of sub-dif f raction-limit imaging techniquesappeared with the need to visualize increasingly smaller elements [2],and with the notion of ultra-microscopy [3]. Several super-resolutionimaging techniques have then been developed in the last century suchas confocal microscopy [4,5] (enhanced further using a double-passconf i guration [6] and the photonic jet phenomenon [7]), scanningnear-f i eld optical microscopy [8,9], structured illumination microscopy[10,11] and metamaterials-based superlenses [12,13]. They con-tributed to bringing optical nanoscopy to the forefront, reinforced bythe Nobel Prize for Chemistry in 2014 with super-resolved f l uorescencemicroscopy [14] and single molecule localization microscopy [15].Nevertheless, these super-resolution imaging techniques require highstability systems, complex alignment or long acquisition time.In 2011, Wang et al. experimentally demonstrated full-f i eld label-free super-resolution microscopy using low-refractive-index micro-spheres (silica microspheres with n ∼1.46) [16]. Relatively easy-to-implement, this approach consists in introducing a transparent micro-sphere in a classical white-light microscope, i.e. between the sampleand the microscope objective. A magnif i ed image, providing sub-dif-fraction-limit information, is then generated and collected by the mi-croscope. In the past years, several papers have focused on the per-formance of the super-resolution imaging technique. In 2012,Darafsheh et al. suggested placing high-refractive-index microspheres(barium-titanate-glass with n∼ 2.0) in an immersion liquid [17] and,more recently, embedding in an elastomer layer [18]. Studies of theimmersion medium inf l uence on the image contrast [19,20] and on theimage nature [21,22], as well as of the role of the coherence of light onhttps://doi.org/10.1016/j.optlastec.2019.01.030Received 7 August 2018; Received in revised form 7 December 2018; Accepted 15 January 2019? Corresponding author.E-mail address: stephane.perrin@unistra.fr (S. Perrin).Optics and Laser Technology 114 (2019) 40–43Available online 25 January 20190030-3992/ © 2019 Elsevier Ltd. All rights reserved.T微球辅助显微术中非常规的放大行为